There are a series of commercially available polyurethane materials which are suitable for the manufacture of implantable medical devices. Typically these materials combine good elasticity, ultimate tensile strength, biocompatibility and biostability. However, only a small percentage of the spectrum of commercially available polyurethanes are suitable for implantation. Those that are suitable for implantation are typically based on a polyether or polycarbonate macroglycol together with a diisocyanate and a diol or diamine chain extender. Examples of these are described in EP Patent No. 461,375 (Pinchuck) and U.S. Pat. No. 5,621,065 (Pudlinear et al).
Products manufactured from conventional polyurethane materials are processed in two stages. Firstly the macroglycol, the diisocyanate and the chain extender are reacted in specific molar ratios so as to produce a polymer with a linear molecular structure. These systems are subsequently processed by any of a variety of thermomechanical or solvent based processes into geometry's suitable for implantation. Polyurethane polymers with a linear molecular structure lend themselves to processing by these techniques. Polyurethanes with three dimensional molecular structures do not lend themselves to processing by either thermomechanical or solvent techniques, rather these materials must be formed into useful articles as part of the polymerisation reaction.
The chemistry and materials of polyurethane manufacture are well known to those skilled in the art and are described in many publications such as the I.C.I. Polyurethane Handbook by George Woods (2.sup.nd Edition John Wiley & Sons).
Most currently available polyurethane materials are chain extended using a diol or a diamine. Diols react with the isocyanate linkage to generate urethane groups whereas amines react with isocyanates to generate urea linkages. Water can also be employed as a chain extender as it reacts with two isocyanate groups to form a urea linkage and gives off carbon dioxide in the process. The hard segment generated with water chain extenders has a high concentration of urea linkage and is thus stiff and polar. Water however is little used for chain extending biomedical polyurethanes as it generates carbon dioxide in the reaction. The inclusion of water is considered to be undesirable.
Linear polymers may be processed by either thermomechanical or solution processes. Polyurethanes with a high urea content are normally difficult to process thermomechanically and are usually solution processed. As most conventional polyurethanes are processed thermomechanically, by processes like extrusion and injection moulding, most conventional polyurethanes do not have high concentrations of urea linkages. Conventional polyurethanes are thus normally chain extended using short chain diols.
Polycarbonate based polyols have been used in the manufacture of polyurethane elastomers as described in U.S. Pat. No. 5,254,662 (Szycher), where the polycarbonates are represented by the following general formula EQU HO(R.sub.1 O(CO)OR.sub.1).sub.n OH
R.sub.1 represents an aliphatic chain of from 2 to 20 carbon atoms, and n has a value sufficient to generate a molecular weight of from 650 to 3500 molecular weight units. The main difficulty with systems based on these polycarbonates is generating a material that is sufficiently soft and elastomeric for implantation in a soft tissue environment. This issue arises due to the regularity of the polycarbonate polyol structure.
The repeat unit of a typical polyol of the form shown above could be represented as follows: EQU (CH.sub.2 CH.sub.2 CH.sub.2 CH.sub.2 CH.sub.2 CH.sub.2 O--CO--O)
The oxygens of the carbonate linkage are more electron withdrawing than the carbons to which they are bonded. The oxygens therefore carry a slight negative charge while the carbons carry a slightly positive charge. When carbonate linkages of neighbouring chains come into close proximity the positive charges of one chain will interact in an attractive fashion with the negative charges of the other chain. The chain structure of the carbonates employed in U.S. Pat. No. 5,524,662 (Szycher) and similarly in EP 461375 (Pinchuck) contains linkages that allow the chain to take up a linear configuration. These chains containing no side groups to disrupt the regularity of the structure. These features allow the attractive forces associated with the carbonate linkage to dominate the stiffness of the polyol phase. This attraction significantly increases the stiffness characteristics of resulting PCU. The greater the concentration of carbonate linkages in this type of regular polyol phase the stiffer the resulting polyurethane will be.
The C--O linkage of the carbonate oxygen and carbon of the hydrocarbon chain could however have a low energy of rotation. This low energy of rotation should have the effect of softening the polyol phase. This is not observed to be the case because the attractive forces between the carbonates lock these rotational movements in and prevent these movements from having a significant influence on the material softness. Because polycarbonates of the type employed by Szycher and Pinchuck are very regular in structure they generate polyurethanes that are much stiffer than comparative polyether urethanes.
It is an object of the present invention to provide soft, flexible polycarbonate urethane polymers having properties making them suitable for long term implantation.